A piezoelectric/electrostrictive membrane sensor can be used for measuring fluid characteristics such as viscosity, density and concentration by use of the mechano-electric transduction of a piezoelectric/electrostrictive element in which a membranous piezoelectric/electrostrictive body disposed in the sensor is sandwiched between a pair of electrodes. When the piezoelectric/electrostrictive membrane sensor (the piezoelectric/electrostrictive element) oscillates in a fluid, the sensor receives a mechanical resistance due to the viscosity resistance of the fluid, and the electric constant of the piezoelectric/electrostrictive element changes in a constant relation with respect to the mechanical resistance. Therefore, the sensor can detect the electric constant to measure the viscosity of the fluid or the like.
It is to be noted that any prior document having the same theme as or a theme in common with the theme of the present invention discussed later does not seem to be present, however, Patent Documents 1 to 6 can be given as examples of a prior document for reference of the piezoelectric/electrostrictive membrane sensor:
Patent Document 1: JP-A-8-201265
Patent Document 2: JP-A-5-267742
Patent Document 3: JP-A-6-260694
Patent Document 4: JP-A-2005-164495
Patent Document 5: JP-A-2-51023
Patent Document 6: JP-A-8-98884
In recent years, the above piezoelectric/electrostrictive membrane sensor (hereinafter referred simply as the sensor) has the following problems in the same manner as in a semiconductor integrated circuit chip (IC, LSI) or the like.
First, there are such problems that a membranous piezoelectric/electrostrictive body (an insulator) causes dielectric breakdown and that the piezoelectric/electrostrictive membrane sensor cannot operate. In a chip made of silicon, there is a known problem that to speed up the operation, it is essential to form a thin membrane and apply a high voltage (electric field), and hence an oxide membrane (the insulator) easily causes the dielectric breakdown. Similarly, in the piezoelectric/electrostrictive membrane sensor, sometimes the applied voltage is made high by thinning the membrane of the piezoelectric/electrostrictive body, for example, in case of a longitudinally effective oscillating element. This is because the oscillation becomes larger when the applied voltage to the piezoelectric/electrostrictive body made of a thin membrane is increased; thereby the sensitivity of the sensor is effectively increased. However, in this case, the piezoelectric/electrostrictive body easily causes the dielectric breakdown. Consequently, the deterioration of the reliability of the piezoelectric/electrostrictive membrane sensor is incurred. Therefore, the prevention of the dielectric breakdown in this case still remains a theme to be solved.
Moreover, in association with the above dielectric breakdown, breakdown due to static electricity (electrostatic discharge damage) has raised a problem. All matter has electrons in atoms. When articles constituted of such matter, or a person and the article come in contact with (including friction, collision, etc.) or peel from each other, the electrons move, and an electrically unstable state is supposedly brought to generate the static electricity. When the static electricity is generated, with the movement of the electrons, the article or the person receiving the electrons forms a minus pole, and the article or the person discharging the electrons forms a plus pole. In the case of the piezoelectric/electrostrictive membrane sensor, the piezoelectric/electrostrictive body as an insulator receives the electrons from another article, the person or air on the surface of the body, and can be brought into a charged state with the minus pole. Moreover, when the body discharges the electrons (discharges electricity) to the other article, the person or the air from this state, a voltage of several kVs is applied during the discharge, and the piezoelectric/electrostrictive body is sometimes damaged (the electrostatic discharge damage). Furthermore, when dust, dirt or the like charged with the plus pole is attracted to adhere to the surface of the piezoelectric/electrostrictive body in the charged state with the minus pole, a pair of electrodes sandwiching the piezoelectric/electrostrictive body therebetween thus causes short circuit, any desired voltage is not applied to the piezoelectric/electrostrictive body, the oscillation of the sensor becomes unstable. Alternatively, the resonance frequency of the sensor deviates owing to the mass of the dust or the like to cause wrong detection or the like. The characteristics of a fluid or the presence of the fluid can not correctly be measured.